This invention relates to acrylic polyurethane coating compositions and, in particular, to an improved acrylic polyurethane coating composition. More particularly, this invention relates to the incorporation of polytrimethylene carbonate diols and triols into acrylic polyurethane coatings to obtain high gloss acrylic coatings with improved impact resistance, with no significant loss of other properties.
Acrylic polyurethane coating compositions are well known and have been widely used to finish and repair finishes on automobiles and trucks. An early high quality acrylic polyurethane coating composition which has been widely used is described in U.S. Pat. No. 3,558,564. Other acrylic polyurethane compositions are described in U.S. Pat. Nos. 4,131,571 and 4,555,535.
Though these acrylic polyurethane coatings provided improvements over any coatings previously available, problems with paint chipping continue to plague motor vehicle owners. There is a need in the art for compositions with improved impact resistance. Attempts have been made to produce tougher, more chip-resistant paint for automobiles, but these have not been completely satisfactory. One recognized alternative solution is to cover all or parts of the finished surface of the automobile with a protective coating, however the acrylic enamel, acrylic lacquer, or nitrocellulose lacquer typically used on vehicles produce coatings which are difficult to overcoat with protective materials.
A useful protective coating composition would first and foremost be chip- and abrasion-resistant, have good adhesion to the painted surface, be clear, smooth (i.e., without surface roughness) and indistinguishable over the painted surface when applied to the areas being protected.
Various attempts have been made to produce chip-resistant polyurethane protective coatings. See, for example, U.S. Pat. No. 4,254,168. Another example is U.S. Pat. No. 3,875,090 which discloses a high impact and abrasion resistant coating composition comprising a mixture of a partially hydrolyzed vinyl chloride-vinyl acetate copolymer, an alkyd and an isocyanate terminated urethane prepolymer.
Thermosetting compositions characterized by hardness as well as flexibility have been disclosed in the art. U.S. Pat. No. 3,691,258 claims coating compositions resistant to yellowing, solvents, acids, and alkalies, and having the combination of high elasticity and good hardness produced from a mixture of an aminoplast, a polyester, and a mixture which includes one or both of ethylene glycol and 1,2-propanediol, optionally an additional diol component, and an acid mixture which consists essentially of a cyclic dicarboxylic acid and an acyclic dicarboxylic acid. U.S. Pat. No. 4,548,998 discloses an improved high solids, solvent-based, thermosetting coating composition useful on surfaces containing rigid as well as elastomeric substrates in combination, comprising about 60 to 95% by weight acyclic moieties, and optionally up to about 30% by weight cyclic moieties. The preparation of this polyurethane polyol does not entail the use of a polyisocyanate.
The use of various modifiers to attempt to improve impact properties of acrylic polyurethanes has been addressed in the art. Polytrimethylene ether glycol(PTMEG) has been suggested as a modifier for polyurethanes, however at the expense of optimum UV resistance. The addition of glycol adipates to improve impact resistance has been suggested, but results in the reduction of hydrolytic stability. Previously proposed impact modifiers typically result in the loss of other properties.
The preparation of polycarbonate polyols is known in the art. U.S. Pat. No. 4,533,729 discloses a process for preparing amorphous polycarbonate polyols by reacting phosgene, a branched-chain polyhydric alcohol, and a straight chain polyhydric alcohol in the presence of a solvent and in the absence of a catalyst at a temperature of from about 60xc2x0 to 100xc2x0 C. The reaction mixture is then contacted with a catalytic amount of a tertiary amine at reflux temperature for a period of time of at least about 30 minutes. It is suggested the resulting polycarbonate polyol can be used in coating compositions. In JP 64001724 there is disclosed the preparation of a polycarbonate polyol from (di) allyl-, alkyl- or alkylene carbonate and a polyhydroxy compound using a titanium catalyst.
The use of a polycarbonate in a polyurethane composition is known. U.S. Pat. No. 5,569,707 discloses aqueous polyester-polyurethane dispersions which are aqueous binders suitable for use in flexible lacquer, coating and sealing compositions, and, in particular, for the production of soft feel coatings. These compositions include 4-75% of a difunctional polyol which can be a polycarbonate. Another reference, U.S. Pat. No. 5,527,879, discloses a coating composition containing a polycarbonate polyol and a melamine resin curing agent which is claimed to provide good solubility in solvent, high compatibility with curing agent, preferred heat and water resistance and good dynamic and physical properties. In U.S. Pat. No. 5,219,663 there is disclosed an aromatic polycarbonate polyurethane resin suitable as a coating agent or film on a magnetic layer of a magnetic recording medium.
JP 08269394A discloses an intermediate coating paint composition containing a polyurethane resin, a polycarbonate diol, a polyisocyanate and a microgel.
The preparation of trimethylene carbonate is known. U.S. Pat. No. 5,212,321 discloses a process for preparing trimethylene carbonate wherein 1,3-propanediol is reacted with diethylcarbonate in the presence of zinc powder, zinc oxide, tin powder, tin halide or organo-tin compound, at an elevated temperature. It is also known in the art to use polytrimethylene carbonate in polyester applications. See U.S. Pat. Nos. 5,225,129 and 5,849,859.
There does not appear to be any reference in the art which discloses or suggests the use of polytrimethylene carbonate diols and triols in relatively small amounts to provide improved impact resistance in acrylic polyurethane compositions.
There is still a need in the art for an acrylic polyurethane coating composition with improved impact resistance. It would be extremely valuable if it were possible to provide improved impact resistance without diminishing or sacrificing any other properties, particularly gloss.
In the present invention it has been found that modified acrylic polyurethane coatings with high impact resistance and high gloss can be achieved through incorporation of polytrimethylene carbonate diols and triols. Furthermore, these improvements were observed while maintaining high gloss, weather resistance, and overall durability. It has surprisingly been found that to have both high impact resistance and high gloss polytrimethylene carbonate diols and triols within a specific molecular weight range provide the best results.
In accordance with the foregoing, the present invention comprises: A curable modified acrylic polyurethane coating composition characterized by improved impact resistance which comprises:
a) An acrylic polyol dissolved in a suitable solvent to 40-90% solids;
b) 2 to 50% by weight of said acrylic polyol substituted with a polytrimethylene carbonate polyol selected from a polytrimethylene carbonate diol, a polytrimethylene carbonate triol, or a higher functionality polytrimethylene carbonate polyol; and
c) A polyisocyanate crosslinking agent;
d) Optionally a catalyst; and
e) Optionally pigments and other additives commonly used in coatings.
The composition can be applied over a wide variety of substrates such as plastic, metal, wood, primed metals, or previous coated or painted metals.